Whats the name of the value that you add or subtract from a minimax tree node?
I think you mean the leave nodes only which are changed. The other nodes in the tree are calculated during calculating the best move with this tree. The values at the leaves are called utility values in Russel and Norvig's "Artificial intelligence: a modern approach". Some times it is called heuristic value; see https://en.wikipedia.org/wiki/Minimax.